Multilayer ceramic substrate and method for producing the same

ABSTRACT

When a multilayer ceramic substrate with a cavity is reduced in thickness, a bottom wall portion defining the bottom of the cavity is reduced in thickness, thereby leading to the problem that the bottom wall portion is likely to be broken. A bottom wall portion defining a cavity of a multilayer ceramic substrate has a stack structure formed with a high thermal expansion coefficient layer sandwiched between first and second low thermal expansion coefficient layers. This configuration generates compression stress in the low thermal expansion coefficient layers during a cooling process after firing, thereby allowing the mechanical strength at the bottom wall portion to be improved.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a multilayer ceramic substrate and amethod for producing the multilayer ceramic substrate, and moreparticularly, to an improvement to increase the strength of a multilayerceramic substrate including a cavity.

2. Description of the Related Art

Known methods for producing a multilayer ceramic substrate include, forexample, a method described in Japanese Patent Application Laid-Open No.2003-273513. In Japanese Patent Application Laid-Open No. 2003-273513,in order to solve the problem that a relatively high degree of shrinkagemay occur at a location further away from an open end of a cavity, inwhich the shrinkage suppression effect produced by an outer constraininglayer is weakened, to undesirably deform the cavity when anon-shrinkable process is used to produce a multilayer ceramic substrateincluding the cavity, a firing step is performed with a raw stacked bodysandwiched between outer constraining layers including an inorganicmaterial powder to suppress shrinkage while forming a constraininginterlayer including the inorganic material powder to suppress shrinkagealong a ceramic green layer of the raw stacked body to define amultilayer ceramic substrate, which is located at a location at whichthe cavity formed.

According to the production method described in Japanese PatentApplication Laid-Open No. 2003-273513, in the firing step, the shrinkagesuppression effect produced by the constraining interlayer acts inaddition to the shrinkage suppression effect produced by the outerconstraining layer, thereby substantially preventing shrinkage in thedirection of the principal surface of the ceramic green layer, andenabling a multilayer ceramic substrate to be obtained withoutundesirable deformations of the cavity.

However, the multilayer ceramic substrate including a cavity has aproblem in that a bottom wall portion defining the bottom of the cavityis likely to crack or break.

As the size of electronic devices including a multilayer ceramicsubstrate is reduced, the thickness of the multilayer ceramic substrateis required to be reduced. Therefore, particularly in the case of amultilayer ceramic substrate including a cavity, the thickness of thebottom wall portion must be reduced to achieve the reduction in thethickness of the multilayer ceramic substrate when the size of a mountedcomponent to be disposed in the cavity is determined. Alternatively,when a peripheral portion defining the periphery of the cavity must beincreased in height in order to accommodate mounted components havingvarious sizes and shapes in the cavity, the bottom wall portion must bemade thinner due to the increase in the height of the peripheral wallportion. As a result of these circumstances, the bottom wall portion islikely to be broken, and preventing such a break is a big issue.

In addition, the multilayer ceramic substrate including a cavity doesnot have a uniform thickness, i.e., the multilayer ceramic substrate hasa relatively thin bottom wall portion defining the bottom of the cavityand a relatively thick peripheral wall portion defining the periphery ofthe cavity, makes it more likely to have undesirable deformations, suchas warpage caused by firing. In this case, depending on the relationshipbetween the thickness of the bottom wall portion and the height of theperipheral wall portion, deformations, such as warpage, may be moresignificantly produced in some cases. Therefore, when deformations, suchas warpage, are to be prevented, the degree of design freedom of themultilayer ceramic substrate may be severely limited.

SUMMARY OF THE INVENTION

To overcome the problems described above, preferred embodiments of thepresent invention provide a multilayer ceramic substrate in whichbreakage of a bottom wall portion defining the bottom of a cavity isless likely to occur and in which undesirable deformations, such aswarpage, are effectively suppressed, and a method for producing themultilayer ceramic substrate.

A preferred embodiment of the present invention provides a multilayerceramic substrate which includes a cavity, including a peripheral wallportion including a first ceramic layer including a through holearranged to define the cavity, and a bottom wall portion includingsecond ceramic layers not including a through hole. In the multilayerceramic substrate, the bottom wall portion includes at least two typesof ceramic layers, the at least two types of ceramic layers including ahigh thermal expansion coefficient layer having a relatively highthermal expansion coefficient and a plurality of low thermal expansioncoefficient layers having a relatively low thermal expansioncoefficient, wherein at least a portion of the high thermal expansioncoefficient layer is sandwiched between a first low thermal expansioncoefficient layer and a second low thermal expansion coefficient layerof the plurality of low thermal expansion layers.

An outer surface of the bottom wall portion is preferably defined by thefirst low thermal expansion coefficient layer, and a surface of thebottom wall portion arranged in contact with the peripheral wall portionis preferably defined by the second low thermal expansion coefficientlayer.

The peripheral wall portion preferably includes a high thermal expansioncoefficient layer having a thermal expansion coefficient greater thanthat of the second low thermal expansion coefficient layer, and a thirdlow thermal expansion coefficient layer having a relatively low thermalexpansion coefficient defining the outermost layer of the peripheralwall portion.

The bottom wall portion preferably further includes a first constraininginterlayer arranged in contact with the second low thermal expansioncoefficient layer. In this case, the first constraining interlayerpreferably includes an inorganic material powder which is notsubstantially sintered at a firing condition at which ceramic materialincluded in the low thermal expansion coefficient layer is sintered, andthe inorganic material powder is solidified by permeation of the ceramicmaterial included in the low thermal expansion coefficient layer.However, the first constraining interlayer is not limited to beingsandwiched between the low thermal expansion coefficient layers, andother arrangements may be provided.

The peripheral wall portion preferably further includes a secondconstraining interlayer arranged along a surface of the peripheral wallportion in contact with the bottom wall portion. In this case, thesecond constraining interlayer includes an inorganic material powderwhich is not substantially sintered at a firing condition at which aceramic material included in the low thermal expansion coefficient layeris sintered, and the inorganic material powder is solidified bypermeation of the ceramic material included in the low thermal expansioncoefficient layer.

A through hole included in the second constraining interlayer preferablydoes not extend outwardly from an inner peripheral edge of the throughhole in the first ceramic layer, the through hole included in the firstceramic layer of the peripheral wall portion is arranged in contact withthe second constraining interlayer, and at least a portion of the innerperipheral edge defining the through hole in the second constraininginterlayer is preferably located inwardly of the inner peripheral edgedefining the through hole in the first ceramic layer of the peripheralwall portion arranged in contact with the second constraininginterlayer.

Another preferred embodiment of the present invention provides a methodfor producing a multilayer ceramic substrate including a peripheral wallportion including a first ceramic layer including a through holearranged to define a cavity, and a bottom wall portion including aplurality of second ceramic layers not including a through hole, whereinthe bottom wall portion includes at least two types of ceramic layers,the at least two types of ceramic layers including a high thermalexpansion coefficient layer having a relatively high thermal expansioncoefficient and a plurality of low thermal expansion coefficient layershaving a relatively low thermal expansion coefficient, and at least aportion of the high thermal expansion coefficient layer is sandwichedbetween a first low thermal expansion coefficient layer and a second lowthermal expansion coefficient layer of the plurality of low thermalexpansion coefficient layers.

The method for producing a multilayer ceramic substrate according tothis preferred embodiment of the present invention includes the steps ofpreparing a first ceramic green layer including the through hole, thefirst ceramic green layer to be subjected to firing to form the firstceramic layer, and the first ceramic green layer including alow-temperature sintering ceramic material, preparing, as second ceramicgreen layers to be subjected to firing to form as the second ceramiclayers, a high thermal expansion coefficient green layer to form thehigh thermal expansion coefficient layer, a first low thermal expansioncoefficient green layer to form the first low thermal expansioncoefficient layer, and a second low thermal expansion coefficient greenlayer to form the second low thermal expansion coefficient layer, eachof the green layers including a low-temperature sintering ceramicmaterial, producing a stacked composite body including a raw stackedbody formed by stacking the first ceramic green layer and the secondceramic green layer, and an outer constraining layer provided on bothprincipal surfaces of the raw stacked body, the outer constraining layerincluding an inorganic material powder which is not substantiallysintered at a firing condition at which the low-temperature sinteringceramic material is sintered; firing the stacked composite body at afiring condition at which the low-temperature sintering ceramic materialis sintered; and then removing the outer constraining layers from thestacked composite body.

The production method according to a preferred embodiment of the presentinvention is preferably used to produce a multilayer ceramic substratein which an outer surface of the bottom wall portion is defined by thefirst low thermal expansion coefficient layer, and a surface of thebottom wall portion arranged in contact with the peripheral wall portionis defined by the second low thermal expansion coefficient layer. Inthis case, the raw stacked body preferably further includes, as thesecond ceramic green layer, a first constraining interlayer arranged incontact with the second low thermal expansion coefficient green layer.The first constraining interlayer preferably includes an inorganicmaterial powder which is not substantially sintered at a firingcondition at which the low-temperature sintering ceramic material issintered, and the inorganic material powder is solidified by permeationof the ceramic material included in the low thermal expansioncoefficient green layer as a result of the firing step.

In addition, the raw stacked body preferably further includes, as thefirst ceramic green layer, a second constraining interlayer arrangedalong a surface of the peripheral wall portion in contact with thebottom wall portion. In this case, the second constraining interlayerincludes an inorganic material powder which is not substantiallysintered at a firing condition at which a ceramic material included inthe low thermal expansion coefficient layer is sintered, and theinorganic material powder is solidified by permeation of the ceramicmaterial included in the low thermal expansion coefficient layer as aresult of the firing step.

More preferably, in the raw stacked body, the through hole in the secondconstraining interlayer is made smaller than the through hole in thefirst ceramic green layer of the peripheral wall portion arranged incontact with the second constraining interlayer.

According to various preferred embodiments of the present invention, thebottom wall portion of the cavity has the stack structure including atleast a portion of the high thermal expansion coefficient layersandwiched between the first and second low thermal expansioncoefficient layers, and the first and second low thermal expansioncoefficient layers have compression stress generated during a coolingprocess after the firing. As a result, the strength at the bottom wallportion can be improved, thus making the bottom wall portion less likelyto be broken.

In particular, when the outer surface of the bottom wall portion isdefined by the first low thermal expansion coefficient layer, and whenthe surface of the bottom wall portion in contact with the peripheralwall portion is defined by the second low thermal expansion coefficientlayer, the compression stress generated in the first and second lowthermal expansion coefficient layers will act over substantially theentire thickness of the bottom wall portion, thereby enabling thestrength of the entire bottom wall portion to be effectively improved.

The peripheral wall portion including the high thermal expansioncoefficient layer and the third low thermal expansion coefficient layerdefining the outermost layer of the peripheral wall portion improves thestrength of the entire multilayer ceramic substrate, and greatlysuppresses warpage caused by the stress difference between the front andback of the multilayer ceramic substrate.

When the first constraining interlayer is arranged in contact with thesecond low thermal expansion coefficient layer, shrinkage at theinterface between the bottom wall portion and the peripheral wallportion is greatly suppressed during the firing, thereby preventingundesirable deformations, such as warpage of the multilayer ceramicsubstrate and cracks.

The suppression of undesirable deformations, such as warpage of themultilayer ceramic substrate, increases the degree of design freedom ofthe multilayer ceramic substrate including a cavity.

In the method for producing a multilayer ceramic substrate according tovarious preferred embodiments of the present invention, the stackedcomposite body to be subjected to firing includes the raw stacked bodyto form the multilayer ceramic substrate as well as the outerconstraining layers, and shrinkage of the raw stacked body is greatlysuppressed during the firing. As a result, the dimensional accuracy ofthe multilayer ceramic substrate is improved, and undesirabledeformations, such as warpage, are prevented.

When the raw stacked body includes the first constraining interlayer,shrinkage is suppressed at a boundary between the bottom wall portionand the peripheral wall portion, undesirable deformations and cracks areprevented which may be caused at this site, and the dimensional accuracyis further improved.

In addition, when the raw stacked body further includes the secondconstraining interlayer arranged along the surface of the peripheralwall portion in contact with the bottom wall portion in the multilayerceramic substrate, shrinkage is suppressed at a boundary between thebottom wall portion and the peripheral wall portion, and deformationsand cracks which may occur at this location are effectively prevented.

In addition, when the through hole in the second constraining interlayeris made smaller than the through hole in the first ceramic green layerof the peripheral wall portion in contact with the second constraininginterlayer, it is possible to increase the possibility that at least aportion of the inner peripheral edge defining the through hole in thesecond constraining interlayer can be located inwardly of the innerperipheral edge defining the through hole in the first ceramic greenlayer of the peripheral wall portion in contact with the secondconstraining interlayer, without locating the inner peripheral edgedefining the through hole in the second constraining interlayeroutwardly of the inner peripheral edge defining the through hole in thefirst ceramic green layer of the peripheral wall portion in contact withthe second constraining interlayer, even if a deviation of the locationof the through holes occurs between the second constraining interlayerand the first ceramic green layer in contact with the secondconstraining interlayer during the production of the raw stacked body.Therefore, modifications, cracks, defects, etc., can be suppressed andprevented with more certainty in the bottom wall portion of the cavityafter the firing.

The above and other elements, features, steps, characteristics andadvantages of the present invention will become more apparent from thefollowing detailed description of the preferred embodiments withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating a functional module 31including a multilayer ceramic substrate 1 according to a firstpreferred embodiment of the present invention.

FIG. 2 is a cross-sectional view schematically illustrating a stackedcomposite body 41 produced in the process of manufacturing themultilayer ceramic substrate 1 shown in FIG. 1.

FIG. 3 is a cross-sectional view illustrating a functional module 31including a multilayer ceramic substrate 1 a according to a secondpreferred embodiment of the present invention.

FIG. 4 is a cross-sectional view schematically illustrating a stackedcomposite body 41 a produced in the process of manufacturing themultilayer ceramic substrate 1 a shown in FIG. 3.

FIG. 5 is a cross-sectional view schematically illustrating a multilayerceramic substrate 1 b according to a third preferred embodiment of thepresent invention.

FIG. 6 includes cross-sectional views schematically illustratingmultilayer ceramic substrates 61 to 65 according to Comparative Examples1 to 3 and Examples 1 and 2, which were produced in an experimentalexample performed to confirm an advantageous effect of a preferredembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a cross-sectional view illustrating a functional module 31including a multilayer ceramic substrate 1 according to a firstpreferred embodiment of the present invention.

The multilayer ceramic substrate 1 includes a cavity 3 provided therein.The multilayer ceramic substrate 1 includes a bottom wall portion 4defining the bottom of the cavity 3 and a peripheral wall portion 5defining the periphery of the cavity 3.

The multilayer ceramic substrate 1 includes a stack structure includinga plurality of first ceramic layers 2 a including a through holearranged to define the cavity and a plurality of second ceramic layers 2b which do not include a through hole, and the peripheral wall portion 5includes the first ceramic layers 2 a, whereas the bottom wall portion 4includes the second ceramic layers 2 b. The stacking of the firstceramic layers 2 a including the through hole and the second ceramiclayers 2 b which do not include a through hole define the concave cavity3 in the multilayer ceramic substrate 1. In addition, the bottom wallportion 4 includes a first high thermal expansion coefficient layer 6which has a relatively high thermal expansion coefficient and first andsecond low thermal expansion coefficient layers 7 and 8 which haverelatively low thermal expansion coefficients, wherein at least aportion of the first high thermal expansion coefficient layer 6 issandwiched between the first and second low thermal expansioncoefficient layers 7 and 8. In particular, in the present preferredembodiment, the outer surface of the bottom wall portion 4 includes thefirst low thermal expansion coefficient layer 7, and the surface of thebottom wall portion 4 in contact with the peripheral wall portion 5includes the second low thermal expansion coefficient layer 8.

The peripheral wall portion 5 includes a second high thermal expansioncoefficient layer 9, which has a higher thermal expansion coefficientthan the second low thermal expansion coefficient layer 8, and a thirdlow thermal expansion coefficient layer 10 which has a relatively lowthermal expansion coefficient defining the outermost layer.

In addition, the bottom wall portion 4 preferably includes a firstconstraining interlayer 11 arranged in contact with the second lowthermal expansion coefficient layer 8. In the present preferredembodiment, the first constraining interlayer 11 is preferablysandwiched between two of the second low thermal expansion coefficientlayers 8. In addition, a second constraining interlayer 12 is preferablyprovided along the surface of the peripheral wall portion 5 in contactwith the bottom wall portion 4. It is to be noted that the firstconstraining interlayer 11 may preferably be arranged so as to besandwiched between the second low thermal expansion coefficient layer 8and the first high thermal expansion coefficient layer 6.

The multilayer ceramic substrate 1 preferably includes a plurality ofwiring conductors. The wiring conductors preferably define, for example,passive elements, such as capacitors or inductors, or wiringconnections, such as an electrical connection between elements, andtypically include conductor films 13 to 16 and via hole conductors 17,as shown in FIG. 1.

The conductor films 13 are preferably arranged inside the multilayerceramic substrate 1. The conductive films 14 and are preferably arrangedon principal surfaces of the multilayer ceramic substrate 1. Theconductor films 16 are preferably arranged on the bottom of the cavity3. The via hole conductors 17 are preferably arranged to pass throughspecific ones of the ceramic layers 2 a and 2 b in the thicknessdirection while being electrically connected to respective ones of theconductor films 13 to 16.

Chip components 18 and 19 electrically connected to the externalconductor films 14 are preferably mounted on one principal surface ofthe multilayer ceramic substrate 1. FIG. 1 shows bump electrodes 20arranged to electrically connect the chip component 19 to the externalconductor films 14.

In addition, a chip component 21 electrically connected to the cavitybottom conductor films 16 is preferably mounted inside the cavity 3.FIG. 1 shows bump electrodes 22 arranged to electrically connect thechip component 21 to the cavity bottom conductor films 16.

As described above, the chip components 18, 19, and 21 are preferablymounted on the multilayer ceramic layer 1 to define the functionalmodule 31. The external conductor films 15 arranged on the otherprincipal surface of the multilayer ceramic substrate 1 preferablydefine electrical connections arranged to mount the functional module 31onto a motherboard, not shown.

The multilayer ceramic substrate 1 described above is preferablymanufactured, for example, as follows.

FIG. 2 shows a cross-sectional view illustrating a stacked compositebody 41 produced in the process of manufacturing the multilayer ceramicsubstrate 1. The stacked composite body 41 includes a raw stacked body42 to be subjected to firing to define the multilayer ceramic substrate1, and first and second outer constraining layers 43 and 44 provided onthe both principal surfaces of the raw stacked body 42. It is to benoted that the conductor films 13 to 16 and the via hole conductors 17are omitted in FIG. 2.

With reference to FIG. 2 along with FIG. 1 for explanations, the rawstacked body 42 includes a bottom wall portion 4 defining the bottom ofa cavity 3 and a peripheral wall portion 5 defining the periphery of thecavity 3, as in the multilayer ceramic substrate 1.

The bottom wall portion 4 of the raw stacked body 42 includes secondceramic green layers defining the second ceramic layers 2 b, a firsthigh thermal expansion coefficient green layer 46 to define the firsthigh thermal expansion coefficient layer 6, a first low thermalexpansion coefficient green layer 47 to define the first low thermalexpansion coefficient layer 7, and a second low thermal expansioncoefficient green layer 48 to define the second low thermal expansioncoefficient layer 8. The peripheral wall portion 5 of the raw stackedbody 42 includes first ceramic green layers to define the first ceramiclayers 2 a, a second high thermal expansion coefficient green layer 49to define the second high thermal expansion coefficient layer 9 and athird low thermal expansion coefficient green layer 50 to define thethird low thermal expansion coefficient layer 10. These green layers 46to 50 include a low-temperature sintering ceramic material.

In addition, the raw stacked body 42 includes a first constraininginterlayer 11 defining the second ceramic green layer and a secondconstraining interlayer 12 defining the first ceramic green layer. Theconstraining interlayers 11 and 12 include an inorganic material powderwhich is not substantially sintered at firing conditions which sinterthe low-temperature sintering ceramic material.

It is to be noted that while each of the green layers 46 to 50 typicallyincludes multiple layers, as is obvious from the multiple ceramic layers2 a and multiple ceramic layers 2 b shown in FIG. 1, the interfacesbetween these multiple layers are not shown in FIG. 2. In addition, eachof outer constraining layers 43 and 44 may preferably include multiplelayers.

While the raw stacked body 42 is typically formed by stacking multipleceramic green sheets, the raw stacked body 42 may instead be formed byrepeatedly applying a ceramic slurry.

The outer constraining layers 43 and 44 are stacked on both principalsurfaces of the raw stacked body 42, and subjected to pressure bonding,thereby providing the stacked composite body 41. Further, the secondouter constraining layer 44 arranged on the cavity 3 side is providedwith a through hole 51 in communication with the cavity 3.

Next, the stacked composite body 41 is fired at the firing conditionsfor sintering the low-temperature sintering ceramic material describedabove. In this firing step, the inorganic material powder included inthe constraining interlayers 11 and 12 and the outer constraining layers43 and 44 are not sintered, and the constraining interlayers 11 and 12and the outer constraining layers 43 and 44 do not substantially shrink.Therefore, the shrinkage suppression effect produced by the constraininginterlayers 11 and 12 and the outer constraining layers 43 and 44 actson the raw stacked body 42 until the sintered multilayer ceramicsubstrate 1 is obtained. As a result, undesired deformations, such aswarpage, are prevented from occurring in the obtained multilayer ceramicsubstrate 1, and the dimensional accuracy is significantly improved.

Next, the outer constraining layers 43 and 44 are preferably removedfrom the fired stacked composite body 41, by applying, for example,ultrasonic cleaning or blast processing. The fired outer constraininglayers 43 and 44 are porous, and thus can be easily crushed and removed.

The constraining interlayers 11 and 12 include, as a result of thefiring step, the inorganic material powder that is solidified bypermeation of the material, i.e., a glass component, included in the lowthermal expansion coefficient green layer 48 and/or the high thermalexpansion coefficient green layer 49 adjacent to the constraininginterlayers 11 and 12. It is to be noted that the constraininginterlayers 11 and 12 must have a thickness which allows for thesolidification caused by the permeation of this material.

The multilayer ceramic substrate 1 is obtained in the manner describedabove. The bottom wall portion 4 includes a stack structure including atleast a portion of the first high thermal expansion coefficient layer 6sandwiched between the first and second low thermal expansioncoefficient layers 7 and 8. Accordingly, a compression stress isgenerated in the first and second low thermal expansion coefficientlayers 7 and 8 during a cooling process after the firing step, therebyimproving the mechanical strength of the bottom wall portion 4.

In addition, in this preferred embodiment, the third low thermalexpansion coefficient layer 10 is also preferably provided in whichcompression stress is generated during the cooling process after thefiring step. Therefore, it is possible to suppress undesireddeformations, such as warpage, caused by the stress difference betweenthe front and back of the multilayer ceramic substrate 1.

In the preferred embodiment described above, the low thermal expansioncoefficient layers 7, 8, and 10 each preferably have a thickness ofabout 10 μm to about 100 μm, for example. The reasons are as follows.

The stress caused by the differences in thermal expansion coefficientacts at the interfaces between each of the low thermal expansioncoefficient layers 7, 8, and 10 and each of the high thermal expansioncoefficient layers 6 and 9. More specifically, a compression stress actson the low thermal expansion coefficient layers 7, 8, and 10, and thiscompression stress is decreased with increasing distance from theinterfaces. On the other hand, a tensile stress acts on the high thermalexpansion coefficient layers 6 and 9, and this tensile stress isdecreased with increasing distance from the interfaces. This is becausethe stress is relaxed with increasing distance from the interfaces. Whenthe distance from the interface exceeds about 100 μm, the compressionstress will not significantly act on the interfaces, and the effect ofthe compression stress will not be sufficiently produced. Accordingly,each of the low thermal expansion coefficient layers 7, 8, and 10preferably has a thickness of 100 about μm or less, for example.

On the other hand, when the thicknesses of each the low thermalexpansion coefficient layers 7, 8, and 10 are less than about 10 μm, thehigh thermal expansion coefficient layers 6 and 9 that is decreased instrength by the action the tensile stress will be present in near-outersurface regions located at less than about 10 μm from the outer surfacesof the respective low thermal expansion coefficient layers 7, 8, and 10.Therefore, the near-outer surface regions of the respective high thermalexpansion coefficient layers 6 and 9 are likely to be crushed, andaccordingly, each of the low thermal expansion coefficient layers 7, 8,and 10 preferably have a thickness of about 10 μm or greater, forexample.

While the thicknesses of the high thermal expansion coefficient layers 6and 9 are appropriately determined depending on the thickness of theentire multilayer ceramic substrate 1 and the respective thicknesses ofthe low thermal expansion coefficient layers 7, 8, and 10, the highthermal expansion coefficient layers 6 and 9 preferably have a thicknessof about 10 μm to about 100 μm, for example, after firing step.

In addition, the respective thicknesses of the first and second lowthermal expansion coefficient layers 7 and 8 sandwiching the first highthermal expansion coefficient layer 6 are preferably less than thethickness of the first high thermal expansion coefficient layer 6,because the compression stress can be utilized in an efficient manner.Likewise, the respective thicknesses of the second and third low thermalexpansion coefficient layers 8 and 10 sandwiching the second highthermal expansion coefficient layer 9 are preferably less than thethickness of the second high thermal expansion coefficient layer 9. Inaddition, the first to third low thermal expansion coefficient layers 7,8, and 10 are shown as having the same or substantially the samethickness as each other in FIG. 1, these thicknesses may preferablydiffer from each other in accordance with the design of the multilayerceramic substrate 1, such as the balance between the bottom wall portion4 and the peripheral wall portion 5 and the diameter of the cavity 3.

It is to be noted that while the low thermal expansion coefficient layer7 is shown in FIG. 1 as including three ceramic layers 2 b, thethickness of the low thermal expansion coefficient layer 7 refers to thetotal thickness of the three ceramic layers 2 b. The same applies to therespective thickness of the other low thermal expansion coefficientlayers 8 and 10 and the respective thicknesses of the high thermalexpansion coefficient layers 6 and 9.

The difference in thermal expansion coefficient is preferably set in therange of about 1.0 ppmK⁻¹ to about 4.3 ppmK⁻¹ between the low thermalexpansion coefficient layers 7, 8 and 10 and the high thermal expansioncoefficient layers 6 and 9.

It has been discovered that the difference in thermal expansioncoefficient of about 1.0 ppmK⁻¹ or greater significantly reduces thewarpage of the bottom wall portion 4. More specifically, it has beendiscovered that the amount of warpage is decreased with an increase inthe difference in thermal expansion coefficient in a range in differencein thermal expansion coefficient of less than about 1.0 ppmK⁻¹, and issubstantially constant at about 1.0 ppmK⁻¹ or greater. This is believedto be because the stress acting in an in-plane direction which causeswarpage of the multilayer ceramic substrate 1 is relatively small, ascompared to the stress acting in an in-plane direction of front and backsurfaces due to the difference in thermal expansion coefficient,resulting in correction of the warpage.

On the other hand, the difference in thermal expansion coefficient ofabout 4.3 ppmK⁻¹ or less can, with more certainty, prevent defects, suchas delamination and voids, which are caused by the difference in thermalexpansion coefficient at boundary sections between the low thermalexpansion coefficient layers 7, 8 and 10 and the high thermal expansioncoefficient layers 6 and 9.

The material defining the low thermal expansion coefficient layers 7, 8and 10 preferably includes glass including SiO₂ and MO (provided thatthe MO is at least one selected from CaO, MgO, SrO, and BaO), withSiO₂:MO=23:7-17:13, for example, and the material defining the highthermal expansion coefficient layers 6 and 9 preferably includes glasscontaining SiO₂ and MO, with SiO₂:MO=19:11-11:19, for example.

More preferably, the SiO₂ included in the glass included in the materialdefining the low thermal expansion coefficient layers 7, 8 and 10 isabout 34 weight % to about 73 weight %, for example, and the SiO₂included in the glass included in the material about the high thermalexpansion coefficient layers 6 and 9 is about 22 weight % to about 60weight %, for example.

The preferable compositions and their contents as described above issuitable for the purpose of using a borosilicate glass based material toset to about 1.0 ppmK⁻¹ or more for the difference in thermal expansioncoefficient between the low thermal expansion coefficient layers 7, 8and 10 and the high thermal expansion coefficient layers 6 and 9 and setto about 75 weight % or greater for the ratio by weight of the commoncomponent, for example. The ratio by weight of the common component setto about 75 weight % or greater provides a sufficient joining forcebetween each of the low thermal expansion coefficient layers 7, 8 and 10and each of the high thermal expansion coefficient layers 6 and 9.

The SiO₂ component included in the glass contributes to reducing thethermal expansion coefficient, and the MO component contributes toincreasing in thermal expansion coefficient.

In addition, since the deposition of a moderate amount of crystal fromthe glass in the process of firing is advantageous in terms ofmechanical strength characteristics, the glass composition is preferablycloser to the composition of the deposited crystal. For example, in thecase of SiO₂-MO—Al₂O₃—B₂O₃ based glass, a MAl₂Si₂O₈ or a MSiO₃ crystalare likely to be deposited, and the ratio between SiO₂ and MO is thuspreferably adjusted to so as to be closer to the composition of thecrystal. Therefore, the glass composition for the low thermal expansioncoefficient layers 7, 8 and 10 preferably has a ratio between SiO₂ andMO closer to about 2, for example, in order to reduce the thermalexpansion coefficient, and the glass composition for the high thermalexpansion coefficient layers 6 and 9 preferably has a ratio between SiO₂and MO closer to about 1, for example, in order to increase the thermalexpansion coefficient.

The glass composition for the high thermal expansion coefficient layers6 and 9 preferably has an MO ratio greater than that of the low thermalexpansion coefficient layers 7, 8 and 10, and is thus susceptible tocorrosion during a plating process after the firing, but isinsusceptible to fatal damage because the high thermal expansioncoefficient layers 6 and 9 are not exposed to the surface.

If the glass includes too much SiO₂ in the low thermal expansioncoefficient layers 7, 8 and 10 in order to increase the difference inthermal expansion coefficient, the glass viscosity will not besufficiently decreased during the firing, thereby causing failuresduring sintering. A glass including too much MO results in the inabilityto ensure a sufficient difference in thermal expansion coefficient.

In addition, if the glass includes too much MO in the high thermalexpansion coefficient layers 6 and 9 in order to increase the differencein thermal expansion coefficient, the moisture resistance is decreased,thereby causing insulation failures. A glass including too much SiO₂results in the inability to ensure a sufficient difference in thermalexpansion coefficient.

As described above, the ratio between SiO₂ and MO in the glass ispreferably selected to fall within the range described above, forexample, for each of the low thermal expansion coefficient layers 7, 8and 10 and the high thermal expansion coefficient layers 6 and 9.

The glass included in the material defining the low thermal expansioncoefficient layers 7, 8 and 10 preferably includes about 34 weight % toabout 73 weight % of SiO₂, about 14 weight % to about 41 weight % of MO,about 0 weight % to about 30 weight % of B₂O₃, and about 0 weight % toabout 30 weight % of Al₂O₃, for example, and the glass included in thematerial defining the high thermal expansion coefficient layers 6 and 9preferably includes about 22 weight % to about 60 weight % of SiO₂,about 22 weight % to about 60 weight % of MO, about 0 weight % to about20 weight % of B₂O₃, and about 0 weight % to about 30 weight % of Al₂O₃,for example.

The B₂O₃ provides the glass with a moderate viscosity in order topromote smooth sintering during the firing. If the glass includes toomuch B₂O₃, then the viscosity will be decreased too much, which causesover firing pores to be formed in the surface, thereby resulting ininsulation failures. On the other hand, if the glass includes too littleB₂O₃, the viscosity will be increased, thereby resulting in sinteringfailures.

The Al₂O₃ is a component defining the deposited crystals in the lowthermal expansion coefficient layers 7, 8 and 10. If the glass includestoo much or too little Al₂O₃, the deposition of crystals will be lesslikely to occur.

In addition, the Al₂O₃ improves the chemical stability of the glass, andthus, the plating resistance and the moisture resistance are improved inthe high thermal expansion coefficient layers 6 and 9 which include arelatively large amount of MO. The Al₂O₃ makes an intermediatecontribution between SiO₂ and MO to the thermal expansion coefficient,and the glass including too much Al₂O₃ results in an inability to ensurea sufficient difference in thermal expansion coefficient.

The material defining the low thermal expansion coefficient layers 7, 8and 10 preferably includes 30 weight % to about 60 weight % of Al₂O₃ asa filler, for example, and the material defining the high thermalexpansion coefficient layers 6 and 9 preferably includes about 40 weight% to about 70 weight % of Al₂O₃ as a filler, for example.

The Al₂O₃ filler contributes to increasing the mechanical strength. Thematerial including too little Al₂O₃ filler results in insufficientmechanical strength. In particular, if the high thermal expansioncoefficient layers 6 and 9 on which tensile stress acts have aninsufficient mechanical strength, the high thermal expansion coefficientlayers 6 and 9 will likely break, rendering them unable to produce theeffect from the low thermal expansion coefficient layers 7, 8 and 10reinforced by compression stress. Therefore, the high thermal expansioncoefficient layers 6 and 9 include more Al₂O₃ filler than the lowthermal expansion coefficient layers 7, 8 and 10 so as to increase themechanical strength and withstand a larger difference in thermalexpansion coefficient, and further to produce the effect from thereinforced low thermal expansion coefficient layers 7, 8 and 10.

The Al₂O₃ filler provides an intermediate contribution between the glassin the low thermal expansion coefficient layers 7, 8 and 10 and theglass in the high thermal expansion coefficient layers 6 and 9 to thethermal expansion coefficient, and the material including too much Al₂O₃results in the inability to ensure a difference in thermal expansioncoefficient.

It is to be noted that besides Al₂O₃, for example, other ceramics suchas ZrO₂ may be used as the filler.

It is to be noted that it is not necessary that the first to third lowthermal expansion coefficient layers 7, 8 and have the same compositionas each other or have the same thermal expansion coefficient as eachother, and it is also not necessary that the first and second highthermal expansion coefficient layers 6 and 9 have the same compositionas each other or have the same thermal expansion coefficient as eachother.

More specifically, when the thermal expansion coefficients for each ofthe first and second low thermal expansion coefficient layers 7 and 8are less than the thermal expansion coefficient of the first highthermal expansion coefficient layer 6, the thermal expansion coefficientof the first low thermal expansion coefficient layer 7 and the thermalexpansion coefficient of the second low thermal expansion coefficientlayer 8 may differ from each other. In addition, when the thermalexpansion coefficients for each of the second and third low thermalexpansion coefficient layers 8 and 10 are less than the thermalexpansion coefficient of the second high thermal expansion coefficientlayer 9, the thermal expansion coefficient of the second low thermalexpansion coefficient layer 8 and the thermal expansion coefficient ofthe third low thermal expansion coefficient layer 10 may differ fromeach other. Accordingly, as long as the conditions described above aresatisfied, the respective thermal expansion coefficients can be freelydetermined, and as a result, the degree of design freedom of the cavity3 is improved.

FIGS. 3 and 4 are diagrams describing a second preferred embodiment ofthe present invention, which respectively correspond to FIGS. 1 and 2.In FIGS. 3 and 4, the same reference numerals are assigned to elementscorresponding to the elements shown in FIGS. 1 and 2, and redundantdescriptions are omitted.

A multilayer ceramic substrate 1 a according to the second preferredembodiment includes an inner peripheral edge of a second constraininginterlayer 12 a defining a through hole is located inwardly of an innerperipheral edge of a first ceramic layer 2 a of a peripheral wallportion 5 in contact with the second constraining interlayer 12 a, asshown in FIG. 3.

It is to be noted that it is not necessary that the entire innerperipheral edge of the second constraining interlayer 12 a arranged todefine a through hole (hereinafter, referred to as a “first through holefor cavity formation”) is located inwardly of the inner peripheral edgeof the first ceramic layer 2 a in contact with the second constraininginterlayer 12 a arranged to define a through hole (hereinafter, referredto as a “second through hole for cavity formation”). More specifically,all that is required is that at least a portion of the inner peripheraledge for defining the first through hole for cavity formation is locatedinwardly of the inner peripheral edge for defining the second throughhole for cavity formation.

It is important that the inner peripheral edge for defining the firstthrough hole for cavity formation is not located outwardly of the innerperipheral edge for defining the second through hole for cavityformation. For example, if a portion of the inner peripheral edge fordefining the first through hole for cavity formation is not locatedinwardly of the inner peripheral edge for defining the second throughhole for cavity formation, the portion must be located at least in thesame position as the inner peripheral edge for defining the secondthrough hole for cavity formation.

More specifically, when the first and second through holes for cavityformation have a substantially quadrilateral shape, even when the innerperipheral edge for defining the first through hole for cavity formationis located inwardly of the inner peripheral edge for defining the secondthrough hole for cavity formation on only two sides of thequadrilateral, the inner peripheral edge for defining the first throughhole for cavity formation cannot be located outwardly of the innerperipheral edge for defining the second through hole for cavityformation for the other two sides of the quadrilateral, and must belocated at least in the same location as the inner peripheral edge fordefining the second through hole for cavity formation.

When the multilayer ceramic substrate 1 a according to the secondpreferred embodiment is to be produced, the first through hole forcavity formation is smaller than the second through hole for cavityformation in a raw stacked body 42 provided in a stacked composite body41 a, as shown in FIG. 4, such that at least a portion of the innerperipheral edge for defining the first through hole for cavity formationis located inwardly of the inner peripheral edge for defining the secondthrough hole for cavity formation.

According to the second preferred embodiment, a step of stacking greensheets is performed in many cases for the production of the raw stackedbody 42, and in this stacking step, even if a deviation in position ofthrough holes for cavity formation is undesirably caused between thesecond constraining interlayer 12 a and a first ceramic green layer incontact with the second constraining interlayer 12 a, it is possible toincrease the likelihood that at least a portion of the inner peripheraledge for defining the first through hole for cavity formation can belocated inwardly of the inner peripheral edge for defining the secondthrough hole for cavity formation, without locating the inner peripheraledge for defining the first through hole for cavity formation outwardlyof the inner peripheral edge for defining the second through hole forcavity formation. Therefore, the second preferred embodiment preventsdeformations and cracks in the bottom wall portion 4 of the cavity 3with greater certainty after firing.

FIG. 5 is a cross sectional view illustrating a multilayer ceramicsubstrate 1 b according to a third preferred embodiment of the presentinvention. While the multilayer ceramic substrate 1 b is schematicallyshown in FIG. 5 as compared with FIG. 1, the same reference numerals areassigned to elements corresponding to the elements shown in FIG. 1, andredundant descriptions are omitted.

The multilayer ceramic substrate 1 b shown in FIG. 5 includes noconstraining interlayer. The remaining configuration preferably issubstantially the same as the multilayer ceramic substrate 1 shown inFIG. 1 or the multilayer ceramic substrate 1 a shown in FIG. 3.

Another preferred embodiment of the present invention is the multilayerceramic substrate 1, 1 a, or 1 b that includes no third low thermalexpansion coefficient layer 10.

Next, an experimental example will be described which was performed toconfirm the advantageous effects of preferred embodiments of the presentinvention. In this experimental example, multilayer ceramic substrates61 to 65 according to each of Comparative Examples 1 to 3 and Examples 1and 2 were produced as shown in the cross-sectional views of FIG. 6. InFIG. 6, the same reference numerals are assigned to elementscorresponding to the elements shown in FIG. 1, and redundantdescriptions are omitted.

The multilayer ceramic substrate 65 according to Example 2 has thestructure of the multilayer ceramic substrate 1 shown in FIG. 1.

As compared to the multilayer ceramic substrate 65 according to Example2, the multilayer ceramic substrate 61 according to Comparative Example1 is configured such that all of the ceramic layers defining a bottomwall portion 4 are low thermal expansion coefficient layers 7 and all ofceramic layers defining a peripheral wall portion 5 are low thermalexpansion coefficient layers 10.

The multilayer ceramic substrate 62 according to Comparative Example 2is configured such that all of the ceramic layers defining a bottom wallportion 4 are high thermal expansion coefficient layers 6 and all of theceramic layers defining a peripheral wall portion 5 are high thermalexpansion coefficient layers 9.

The multilayer ceramic substrate 63 according to Comparative Example 3is configured such that a bottom wall portion 4 includes no second lowthermal expansion coefficient layers 8 and high thermal expansioncoefficient layers 6 are provided for the second low thermal expansioncoefficient layers 8.

The multilayer ceramic substrate 64 according to Example 1 includes nothird low thermal expansion coefficient layer 10 and a high thermalexpansion coefficient layer 9 provided for the third low thermalexpansion coefficient layer 10.

In this experimental example, the thermal expansion coefficient was setto about 5.3 ppmK⁻¹ for the low thermal expansion coefficient layers 7,8, and 10. In addition, in order to form the low thermal expansioncoefficient layers 7, 8, and 10, green sheets having a thickness ofabout 50 μm were produced, and an appropriate number of the green sheetswere stacked to provide a desired thickness as will be described later.

The green sheets for the low thermal expansion coefficient layers 7, 8,and 10 include a borosilicate based glass powder and a ceramic powder ata ratio by weight of about 60:40, which were obtained by adding about 50parts by weight of an organic solvent, about 10 parts by weight of abutyral based binder, and about 1 part by weight of a plasticizer toabout 100 parts by weight of the total of the glass powder and theceramic powder to provide a slurry, removing air bubbles from thisslurry, then forming the slurry into the shape of a sheet in accordancewith a doctor blade method, and drying the formed sheets. Powderincluding about 46 weight % of SiO₂, 30 weight % of B₂O₃, about 14weight % of CaO, about 5 weight % of Al₂O₃, and about 5 weight % of TiO₂was used as the borosilicate based glass powder, and Al₂O₃ powder wasused as the ceramic powder.

The thermal expansion coefficient was set to about 7.7 ppmK⁻¹ for thehigh thermal expansion coefficient layers 6 and 9. In addition, in orderto form the high thermal expansion coefficient layers 6 and 9, greensheets having a thickness of about 50 μm were produced, and anappropriate number of the green sheets were stacked to provide a desiredthickness as will be described later.

The green sheets for the high thermal expansion coefficient layers 6 and9 include a borosilicate glass powder and a ceramic powder at a ratio byweight of about 70:30, which were obtained by adding an organic solvent,a butyral based binder, and a plasticizer at the same ratio as in thecase of the low thermal expansion coefficient layers described above toabout 100 parts by weight of the total of the glass powder and theceramic powder, and going through the same operations. Powder includingabout 40 weight % of SiO₂, about 5 weight % of B₂O₃, about 40 weight %of CaO, about 5 weight % of MgO, and about 10 weight % of Al₂O₃ was usedas the borosilicate glass powder, and Al₂O₃ powder was used as theceramic powder.

In order to form the constraining interlayers 11 and 12, green sheetshaving a thickness of about 10 μm were produced, and in order to formouter constraining layers, not shown in FIG. 6, green sheets having athickness of about 100 μm were produced. The green sheets for theconstraining interlayers 11 and 12 and the outer constraining layersinclude about 100 parts by weight of an alumina powder, about 10 partsby weight of a butyral based binder, and about 1 part by weight of aplasticizer, which were obtained through substantially the sameoperations as in the case of the low thermal expansion coefficientlayers.

As a conductive paste for conductor films and via hole conductors, notshown in FIG. 6, a conductive paste was used that includes about 48parts by weight of a silver powder, about 3 parts by weight of an ethylcellulose binder, and about 49 parts by weight of an organic solvent,terpenes, and this conductive paste was applied to specific green sheetsin order to form the conductor films 13 to 16 and the via holeconductors 17 as shown in FIG. 1.

Next, the various types of green sheets were stacked to the number ofsheets shown in the column “The Number of Green Sheets Used” of thefollowing Table 1, thereby producing raw stacked bodies defining themultilayer ceramic substrates 61 to 65, and outer constraining layerswere formed on the top and bottom of the raw stacked bodies to producestacked composite bodies. In this case, the outer constraining layerswere formed by stacking four of the green sheets having a thickness ofabout 100 μm on each of the bottom and top of the raw stacked bodies.

TABLE 1 The Number of Green Sheets Used Comparative Bottom Wall LowThermal Expansion 18 Example 1 Portion Coefficient Layer ConstrainingInterlayer 1 Peripheral Low Thermal Expansion 18 Wall PortionCoefficient Layer Constraining Interlayer 1 Comparative Bottom Wall HighThermal Expansion 18 Example 2 Portion Coefficient Layer ConstrainingInterlayer 1 Peripheral High Thermal Expansion 18 Wall PortionCoefficient Layer Constraining Interlayer 1 Comparative Bottom Wall LowThermal Expansion 3 Example 3 Portion Coefficient Layer High ThermalExpansion 15 Coefficient Layer Constraining Interlayer 1 Peripheral LowThermal Expansion 3 Wall Portion Coefficient Layer High ThermalExpansion 15 Coefficient Layer Constraining Interlayer 1 Example 1Bottom Wall Low Thermal Expansion 6 (3 + 3) Portion Coefficient LayerHigh Thermal Expansion 12 Coefficient Layer Constraining Interlayer 1Peripheral High Thermal Expansion 18 Wall Portion Coefficient LayerConstraining Interlayer 1 Example 2 Bottom Wall Low Thermal Expansion 6(3 + 3) Portion Coefficient Layer High Thermal Expansion 12 CoefficientLayer Constraining Interlayer 1 Peripheral Low Thermal Expansion 3 WallPortion Coefficient Layer High Thermal Expansion 15 Coefficient LayerConstraining Interlayer 1

Then, the stacked composite bodies were fired at a temperature of about870° C. for about 10 minutes. Next, the porous outer constraining layersattached to the surfaces of the stacked composite bodies fired wereremoved using an ultrasonic cleaning machine to obtain the multilayerceramic substrates 61 to 65 according to Comparative Examples 1 to 3 andExamples 1 and 2.

Next, in order to compare the mechanical strengths of the respectivemultilayer ceramic substrates 61 to 65 against drop impacts, thefollowing test was performed.

Each of the multilayer ceramic substrates 61 to 65 was mounted on amounting board with solder, and the mounting board was attached to theinside of a substantially rectangular parallelepiped housing and droppedtoward a concrete block. In this case, dropping while sequentiallyfacing the respective six surfaces of the housing downward was referredto as 1 cycle, and this test was performed up to 10 cycles. The bottomwall portions 4 of the respective multilayer ceramic substrates 61 to 65were evaluated to determine whether the bottom wall portions werecrushed or cracked. The results are shown in Table 2.

TABLE 2 The Number of Cycles Condition of Bottom Implemented WallPortion Comparative 4 Crushed Example 1 Comparative 4 Crushed Example 2Comparative 7 Cracked Example 3 Example 1 10 Not Crushed or CrackedExample 2 10 Not Crushed or Cracked

As is clear from Table 2, the bottom wall portions 4 of the multilayerceramic substrates 61 and 62 were crushed in the fourth cycle inComparative Examples 1 and 2. In addition, in Comparative Example 3, thebottom wall portion 4 was cracked in the seventh cycle while completecrushing was prevented.

In contrast, no crushing or cracking occurred in 10 cycles in Examples 1and 2.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing the scope andspirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

What is claimed is:
 1. A method for producing a multilayer ceramicsubstrate including a cavity, comprising a peripheral wall portionincluding a first ceramic layer including a through hole arranged todefine the cavity, and a bottom wall portion including a plurality ofsecond ceramic layers not including a through hole, wherein theplurality of second ceramic layers of the bottom wall portion include atleast two types of ceramic layers, the at least two types of ceramiclayers including a high thermal expansion coefficient layer having arelatively high thermal expansion coefficient and a plurality of lowthermal expansion coefficient layers having a relatively low thermalexpansion coefficient, and at least a portion of the high thermalexpansion coefficient layer is sandwiched between a first low thermalexpansion coefficient layer and a second low thermal expansioncoefficient layer of the plurality of low thermal expansion coefficientlayers, the method comprising the steps of: preparing a first ceramicgreen layer including the through hole, the first ceramic green layer tobe subjected to firing to form the first ceramic layer, and the firstceramic green layer including a low-temperature sintering ceramicmaterial; preparing, as a plurality of second ceramic green layers to besubjected to firing to form the second ceramic layers, a high thermalexpansion coefficient green layer to form the high thermal expansioncoefficient layer, a first low thermal expansion coefficient green layerto form the first low thermal expansion coefficient layer, and a secondlow thermal expansion coefficient green layer to form the second lowthermal expansion coefficient layer, each of the plurality of secondceramic green layers including a low-temperature sintering ceramicmaterial; producing a stacked composite body including a raw stackedbody formed by stacking the first ceramic green layer and the secondceramic green layer, and an outer constraining layer provided on bothprincipal surfaces of the raw stacked body, the outer constraining layerincluding an inorganic material powder which is not substantiallysintered at a firing condition at which the low-temperature sinteringceramic material is sintered; firing the stacked composite body under afiring condition at which the low-temperature sintering ceramic materialis sintered; and then removing the outer constraining layers from thestacked composite body.
 2. The method for producing a multilayer ceramicsubstrate according to claim 1, wherein an outer surface of the bottomwall portion is defined by the first low thermal expansion coefficientlayer, and a surface of the bottom wall portion arranged in contact withthe peripheral wall portion is defined by the second low thermalexpansion coefficient layer, the raw stacked body further includes, asthe second ceramic green layer, a first constraining interlayer arrangedin contact with the second low thermal expansion coefficient greenlayer, the first constraining interlayer includes an inorganic materialpowder which is not substantially sintered at a firing condition atwhich the low-temperature sintering ceramic material is sintered, andthe inorganic material powder is solidified by permeation of the ceramicmaterial included in the low thermal expansion coefficient green layeras a result of the firing step.
 3. The method for producing a multilayerceramic substrate according to claim 1, wherein the raw stacked bodyfurther includes, as the first ceramic green layer, a secondconstraining interlayer arranged along a surface of the peripheral wallportion in contact with the bottom wall portion, the second constraininginterlayer includes an inorganic material powder which is notsubstantially sintered at a firing condition at which a ceramic materialincluded in the low thermal expansion coefficient layer is sintered, andthe inorganic material powder is solidified by permeation of the ceramicmaterial included in the low thermal expansion coefficient layer as aresult of the firing step.
 4. The method for producing a multilayerceramic substrate according to claim 3, wherein in the raw stacked body,the through hole included in the second constraining interlayer is madesmaller than the through hole included in the first ceramic green layerof the peripheral wall portion arranged in contact with the secondconstraining interlayer.